body {
    padding: 0;
    margin: 0;
}

#grey {
    width: 400px;
    height: 200px;
    margin: auto;
    background-color: #ccc;

    position: absolute;
    top: 0;
    right: 0;
    bottom: 0;
    left: 0;
    overflow: hidden;
}

.yellow {
    width: 100px;
    height: 100px;
    background-color: #fc0;

    position: absolute;
    border-radius: 50px; /* 50%也可以 */
}

.a {
    top: -50px;
    left: -50px;
}

.b {
    right: -50px;
    bottom: -50px;
}
